This invention relates to a magnetic disk for an index servo system and also to an index servo system for positioning a read/write head, i.e., a transducer, on the magnetic disk.
In the field of small size magnetic disk devices, a simple open-loop positioning system, which employs a stepping motor for driving a magnetic head in the radial direction of the disk, has been used for positioning the head on the disk. With this positioning system, the positioning accuracy depends solely on the head-feeding accuracy of the stepping motor. This means that a thermal off-track error occurs due to thermal expansion and contraction the disk inevitably occurs to a great extent, making it difficult to increase the track density of the disk.
In order to permit accurate positioning of a head on a disk having a high track density, embedded servo systems, have been proposed, which use servo information embedded in part of a disk area and adapted for positioning the head. Such embedded servo systems include a sector servo system, in which a disk has a plurality of servo sectors and the servo information is embedded in each of the servo sectors, and an index servo system, in which the servo information is provided in a single servo area of the disk.
In the servo sector system, the head can be accurately positioned on a data track of the disk irrespective of thermal off-track or eccentricity of the disk because of the provision of a plurality of servo sectors. For this reason, this system is suited for a magnetic disk device of disk exchange type. However, since servo information is embedded in each servo sector, it is impossible to format a disk with a soft-sector interface. This means that it is impossible to use any existing controller.
With the index servo type, on the other hand, since the servo information is formed in the index area alone, it is impossible to follow the eccentricity of the disk. However, in the field of stationary magnetic disk devices which occupy a considerable proportion of small size magnetic disk devices, the index servo system permits high-precision positioning of the head irrespective of the thermal off-track. In addition, it is possible to format the disk using an existing controller. Heretofore, various index servo systems have been proposed.
One such index servo system is an embedded servo system, called "Digilok", as disclosed in U.S. Pat. No. 4,488,187 issued Dec. 11, 1984. According to this Digilok servo technique, head position information is obtained through counting of pulses obtained by reading burst-type servo information embedded in an index area. Thus, accurate servo control can be attained with relatively simple digital circuits. Since pulse counts represent a head position, however, it is necessary to record the burst-like servo information obliquely with respect to tracks. For this reason, a considerable time is required for writing the servo information. In addition, a dropout of servo information will occur a positioning error.
Japanese Laid-Open Patent Publication No. 60-113370 published June 19, 1985 discloses a different index servo technique. In this technique, two burst-like servo information patterns A and B are recorded on adjacent tracks in an index area of a disk such that they will not overlap each other in the extending direction of tracks. Servo patterns A and B are each formed with deviation from a data track of the data area by one half of the track pitch. To obtain head position information, output signals of the head, corresponding to servo information A and B, are rectified and averaged (i.e., integrated). A difference between amplitude values of servo signals corresponding to servo information A and B is calculated, and an off-track amount of the head is obtained according to the difference data. With this technique, the averaging of signal cannot be attained sufficiently if each of the servo patterns A and B has an insufficient bit number (burst number). For this reason, when a bit or bits in a servo pattern are dropped out head positioning errors will occur. Further, signal waveforms obtained from the servo patterns are different between the inner and outer sides of the disk. Therefore, the rectification and averaging of servo information are liable to cause position signal error between the inner and outer sides of the disk.
A copending U.S. patent application Ser. No. 469,144 filed on Feb. 23, 1983, now U.S. Pat. No. 4,499,511 and entitled "SYSTEM FOR DETECTING POSITION OF A READ-WRITE HEAD IN SEEK OPERATION ON A DISK MEMORY HAVING DATA AND SERVO SECTORS", and assigned to the same assignee as this application, discloses an improved servo system for a disk having concentric tracks.
A copending U.S. patent application Ser. No. 586,440 filed on Mar. 5, 1984, entitled "SYSTEM FOR DETECTING THE POSITION OF A READ-WRITE HEAD ON A DISK RECORDING MEDIUM HAVING DATA AND SERVO SECTORS" and assigned to the same assignee as this application discloses an improved servo system for a disk having concentric tracks.
A copending U.S. patent application Ser. No. 807,998 filed on Dec. 12, 1985, now U.S. Pat. No. 4,636,885 entitled "SERVO SYSTEM FOR A MAGNETIC DISK MEMORY HAVING SPIRAL TRACKS" and assigned to the same assignee as this application discloses an improved servo system for a magnetic disk having spiral tracks.